The processing and or utilization of natural water in systems wherein the water is concentrated with or without heating can result in the formation of insoluble salts. These precipitated salts are troublesome in the systems in that they can lead to scale formation that negatively impacts operational efficiency. For example, in the processing of natural waters such as seawater, brackish, and estuarine water by desalination techniques such as thermal multistage flash distillation (MSF) or reverse osmosis to yield potable water, salts including calcium carbonate, magnesium hydroxide, and calcium sulfate have a tendency to precipitate and form scale deposits on the surfaces of the processing equipment. These scale deposits interfere with the operational efficiency of the systems by obstructing water flow, impeding heat transfer, and inducing localized corrosion that can result in unscheduled maintenance shutdowns and shortening of the service life of the equipment.
Numerous methods to mitigate scale formation in fresh water recovery systems are known to those skilled in the art. For examples see Dunseth et al., Ind. & Eng. Chem. 56, 56 (1964) and the background of the invention of Desai in U.S. Pat. No. 4,200,500.
One common method to prevent scale formation in desalination systems is the practice of acid doping. While effective, this method also has disadvantages of accelerating corrosion processes due to the resulting low pH of the natural water, is not effective in controlling calcium sulfate scale, and there are safety issues in handling large quantities of acid.
Another method to prevent scale formation in desalination systems is the addition of additives in the form of polymers to the natural water at levels substoichiometric to the amount of hardness ions present in the natural water.
Salutsky in U.S. Pat. No. 3,444,054 teaches a method for treating saline evaporator waters to prevent scale formation on the heat transfer surfaces by the addition of a water-soluble poly[methacrylic acid]. It is further taught that the saline waters can contain other treatment agents such as antifoam agents, corrosion inhibitors, oxygen scavengers and the like if they are compatible with the poly[methacrylic acid].
Jones et al. in U.S. Pat. No. 3,810,834 teaches a method for controlling scale formation in aqueous systems, including thermal and reverse osmosis processes, by the addition of a water-soluble poly[maleic acid]. An additional benefit noted is that if scale formation does occur the precipitates crystal structures are modified in such a way that they are easily removed. It is further taught that the poly[maleic acid] of the invention can be utilized in conjunction with other conventional water-treatment agents including phosphonate-type threshold agents, antifoams, and corrosion inhibitors.
Desai in U.S. Pat. No. 4,200,500 teaches a method to prevent deposition of calcium and magnesium scale in the desalination of seawater by the addition of substoichiometric amounts of the hydrolyzed reaction product of maleic anhydride and a polyunsaturated long-chain fatty acid component.
Smith et al. in U.S. Pat. Nos. 4,046,707 and 4,105,551 teaches a method of inhibiting the precipitation of scale forming salts comprising adding to the system a telemetric compound derived from polymerization of a vinylic carboxylic acid in the presence of a phosphorous acid chain transfer agent. It is further taught that the telomer of the inventions can be used in conjunction with other compounds know to be useful in water treatment; e.g., dispersing and threshold agents, precipitating agents, oxygen scavengers, sequestering agents, antifoam agents, and corrosion inhibitors.
Hodgson et al. in U.S. Pat. No. 4,204,953 teaches a method for inhibiting deposition of scale from saline water onto the exchanger surface by utilizing a scale inhibiting additive in conjunction with a mineral acid to neutralize part of the bicarbonate alkalinity. Examples of scale inhibiting additives are polyphosphates, poly(meth)acrylates, phosphonates, aminophosphonates, and polymeric carboxylic acids such as poly[maleic acid].
Logan et al. in EP 0089189 teach an inhibitor of the formation of calcium carbonate, magnesium hydroxide, and calcium sulfate scale comprising a mixture of poly[maleic acid], phosphonic acid, and a hydroxy acid iron sequestrants.
Logan et al. in U.S. Pat. No. 4,634,532 teaches a process for controlling the formation of seawater scale comprising adding to the seawater a water-soluble orthophosphate and at least one water-soluble polycarboxylate, phosphonate, or sulfonate copolymer.
Becker in U.S. Pat. Nos. 4,446,028 and 4,446,046; and Boyette et al. in U.S. Pat. No. 5,512,183 teaches homo- and copolymers of isopropenylphosphonic acid. These compositions are taught to be effective in inhibiting corrosion and scale formation in aqueous water systems such as cooling and boiler system. There is no indication that these compositions would be effective in treating desalination operations or that they would be effective in controlling magnesium hydroxide scaling typical of desalination operations.
Cady et al. in U.S. Pat. No. 5,519,102 teach a method for the preparation of homopolymers of isopropenylphosphonic acid in an aqueous solvent.
In spite of these advances operators of desalination facilities continue to utilize acid to control the pH of the system and, for thermal deslination units, limit the operation temperature to minimize concerns of scale deposition. Of particular concern is the formation of magnesium hydroxide scale in thermal desalination operations. There is therefore a need in this field for a composition that can be added to the aqueous system that control the formation of scale forming species such as magnesium hydroxide without the need for acid addition. Furthermore it is desirable to be able to operate thermal desalination units at higher temperatures to improve their efficiency.